UN Secretary-General Antonio Guterres on Thursday condemned Russia's plans to declare the annexation of occupied parts of Ukraine, calling it "a dangerous escalation" that "has no place in the modern world."

"Any decision to proceed with the annexation of Donetsk, Luhansk, Kherson and Zaporizhzhia regions of Ukraine would have no legal value and deserves to be condemned," Guterres said in a statement.

"It must not be accepted."

Earlier Russian President Vladimir Putin's spokesman Dmitry Peskov said that the four regions, large parts of which are occupied by Russian forces, would be formally incorporated into Russia in a Kremlin ceremony on Friday.

Guterres said any annexation of a country's territory based on the use of force violates the UN Charter and international law.

He said much-criticized referendums held by Russia in the occupied regions took place in the middle of a war under Russian armed occupation and outside Ukraine's own constitutional framework.

"They cannot be called a genuine expression of the popular will," he said.

"The Russian Federation, as one of the five permanent members of the Security Council, shares a particular responsibility to respect the Charter," he added.

"Any decision by Russia to go forward will further jeopardize the prospects for peace."